In mammalian cells, DNA double-strand breaks (DSB) can be repaired by 2 main pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). To give access to DNA damage to the repair machinery the chromatin structure needs to be relaxed, and chromatin modifications play major roles in the control of these processes. Among the chromatin modifications, changes in nucleosome composition can influence DNA damage response as observed with the H2A.Z histone variant in yeast. In mammals, p400, an ATPase of the SWI/SNF family able to incorporate H2A.Z in chromatin, was found to be important for histone ubiquitination and BRCA1 recruitment around DSB or for HR in cooperation with Rad51. Recent data with 293T cells showed that mammalian H2A.Z is recruited to DSBs and is important to control DNA resection, therefore participating both in HR and NHEJ. Here we show that depletion of H2A.Z in the osteosarcoma U2OS cell line and in immortalized human fibroblasts does not change parameters of DNA DSB repair while affecting clonogenic ability and cell cycle distribution. In addition, no recruitment of H2A.Z around DSB can be detected in U2OS cells either after local laser irradiation or by chromatin immunoprecipitation. These data suggest that the role of H2A.Z in DSB repair is not ubiquitous in mammals. In addition, given that important cellular parameters, such as cell viability and cell cycle distribution, are more sensitive to H2A.Z depletion than DNA repair, our results underline the difficulty to investigate the role of versatile factors such as H2A.Z.
Myogenesis is an evolutionarily conserved process. Little known, however, is how the morphology of each muscle is determined, such that movements relying upon contraction of many muscles are both precise and coordinated. Each Drosophila larval muscle is a single multinucleated fiber whose morphology reflects expression of distinctive identity Transcription Factors (iTFs). By deleting transcription cis-regulatory modules of one iTF, Collier, we generated viable muscle identity mutants, allowing live imaging and locomotion assays. We show that both selection of muscle attachment sites and muscle/muscle matching is intrinsic to muscle identity and requires transcriptional reprogramming of syncytial nuclei. Live-imaging shows that the staggered muscle pattern involves attraction to tendon cells and heterotypic muscle-muscle adhesion. Unbalance leads to formation of branched muscles, and this correlates with locomotor behavior deficit. Thus, engineering Drosophila muscle identity mutants allows to investigate, in vivo, physiological and mechanical properties of abnormal muscles.
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